The present invention relates to a method for soldering a composite module component having an electronic component (low-heat-resistant electronic component) which cannot be soldered onto a substrate simultaneously with an electronic component (heat-resistant electronic component) by reflowing, or an unwashable electronic component such as a high frequency electronic component containing a crystal oscillator which cannot be soldered onto the substrate simultaneously with a washable electronic component by reflowing.
When in the composite module component, low-heat-resistant electronic components are soldered onto the substrate simultaneously with heat-resistant electronic components by reflowing at a high temperature, solder which has connected the low-heat-resistant electronic components of the composite module component to the substrate is melted. As a result, the electrical characteristic of the composite module component is destroyed. Therefore, the heat-resistant electronic components cannot be soldered onto the substrate simultaneously with the low-heat-resistant electronic components. In electrically connecting the heat-resistant electronic component and the low-heat-resistant electronic component with the substrate, first, the heat-resistant electronic components are soldered all together onto the substrate according to a procedure as shown in FIG. 23. Then, the low-heat-resistant electronic components are manually soldered onto the substrate. According to another method, the heat-resistant electronic components are soldered all together onto the substrate by reflowing and then low temperature solder is applied to the substrate so as to solder the low-heat-resistant electronic components onto the substrate by reflowing at a low temperature as shown in FIG. 24.
In connecting to a substrate washable electronic components and unwashable electronic components having a crystal oscillator which may be destroyed when it is subjected to ultrasonic waves, the washable electronic components are soldered simultaneously onto the substrate by reflowing and washed. Then, the unwashable electronic components are manually soldered onto the substrate as shown in FIG. 25.
The above-described manual soldering of the low-heat-resistant electronic components and the unwashable electronic components are performed to decrease the action of flux and relieve the heat shock of a printed circuit board. That is, as shown in FIG. 26, electrodes 100a and 100b of a substrate 34, a heat-resistant composite module component 99, and an electronic unwashable component 103 having a crystal oscillator 101 are preheated. Then, solder 97 is supplied and melted by a soldering iron 98. Then, the solder 97 is cause to flow onto connecting portions. Thereafter, the heat-resistant electronic component 99 and the unwashable electronic component 103 are supplied to a substrate 34 and the solder 97 is then melted by the soldering iron 98. Thus, the electrodes of the electronic component 99 are soldered onto the electrodes 100a and 100b of the substrate 34.
However, it depends on an operator's workmanship as to whether or not electronic components are soldered favorably onto the substrate. Thus, the conventional soldering method is not suitable for mass production.
When the electronic components are electrically connected with the substrate in manual operation, fine solder balls scatter in melting the solder 97. When the solder balls 102 stay on an electronic circuit, the function thereof is damaged. In the case of the washable electronic components, the solder balls can be removed from the substrate by washing the substrate and the electronic components simultaneously after they have been soldered onto the substrate, but in the case of the unwashable electronic components having the crystal oscillator, the solder balls cannot be removed from the substrate because the components are unwashable.
In connecting the low-heat-resistant electronic component with the substrate by reflowing at a low temperature, the melting point of the low-temperature solder is approximately 150.degree. C. while the melting point of eutectic solder is 183.degree. C. Therefore, it is difficult to maintain the same temperature and, as such, soldering cannot be accomplished with the same quality.